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Independence densities of hypergraphs
Anthony BonatoRyerson University
2014 CMS Summer Meeting
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Paths
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…• number of
independent sets = F(n+2)
- Fibonacci numberPn
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Stars
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• number of independent sets = 1+2n
• independence density
= 2-n-1+½
K1,n
…
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Independence density
• G order n
• i(G) = number of independent sets in G (including ∅)
– Fibonacci number of G
• id(G) = i(G) / 2n
– independence density of G
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Properties
• if G is a spanning subgraph of H, then
i(H) ≤ i(G)
• i(G U H) = i(G)i(H)
• if G is subgraph of H, then id(H) ≤ id(G)– G has an edge, then: id(G) ≤ id(K2) = 3/4
• id(G U H) = id(G)id(H)
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Infinite graphs?
• view countably a infinite graph as a limit of chains
• extend definition by continuity:
• well-defined?• possible values?
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Chains
• let G be a countably infinite graph
• a chain C in G is a set of induced subgraphs Gi such that:– for all i, Gi is an induced subgraph of Gi+1 and
• write id(G, C) =
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Existence and uniqueness
Theorem (B,Brown,Kemkes,Pralat,11)
Let G be a countably infinite graph.
1. For each chain C, id(G, C) exists.
2. For all chains C and C’ in G,
id(G, C)=id(G, C’).
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Examples
• stars: id(K1,∞) = 1/2
• one-way infinite path: id(P∞) = 0
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Bounds on id
• if G contains and infinite matching, then id(G) = 0• matching number of G, written µ(G), is the supremum of
the cardinalities of pairwise non-intersecting edges in G
Theorem (B,Brown,Kemkes,Pralat,14)
If µ(G) is finite, then:
• in particular, id(G) = 0 iff µ(G) is infinite
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Rationality
Theorem (BBKP,11)
Let G be a countable graph.
1. id(G) is rational.
2. The closure of the set
{id(G): G countable}
is a subset of the rationals.
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Aside: other densities
• many other density notions for graphs and hypergraphs:– upper density– homomorphism density– Turán density– co-degree density– cop density, …
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Question
• hereditary graph class X: closed under induced subgraphs
• egs: X = independent sets; cliques; triangle-free graphs;
perfect graphs; H-free graphs
• Xd(G) = proportion of subsets which induce a graph in X– generalizes to infinite graphs via chains
• Is Xd(G) rational?
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Hypergraphs
• hypergraph H = (V,E), E = hyperedges• independent set: does not contain a
hyperedge
• id(H) defined analogously– extend to infinite hypergraphs by continuity– well-defined
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1
2 3
4
Examples
∅,{1},{2},{3},{4},{1,2},{1,3},{2,3},{1,4},{3,4},{1,3,4}
id(H) = 11/16H
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Examples, cont
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…
id(H) = 7/8
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Hypergraph id’s
examples:
1. graph, E = subsets of vertices containing a copy of K2
– recovers the independence density of graphs
2. graph, fix a finite graph F; E = subsets of vertices containing a copy of F– F-free density (generalizes (1)).
3. relational structure (graphs, digraphs, orders, etc); F a set of finite structures; E = subsets of vertices containing a member of F – F-free density of a structure (generalizes (2))
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Bounds on id
• matching number of hypergraph H, written µ(H), is the supremum of the cardinalities of pairwise non-intersecting hyperedges in H
Theorem (B,Brown,Mitsche,Pralat,14)
Let H be a hypergraph whose hyperedges have cardinality bounded by k > 0. If µ(H) is finite, then:
• sharp if k = 1,2; not sure lower bound is sharp if k > 2
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Rationality
• rank k hypergraph: hyperedges bounded in cardinality by k > 0– finite rank: rank k for some k
Theorem (BBMP,14): If H has finite rank,
then id(H) is rational.
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Sketch of proof
• notation: for finite disjoint sets of vertices A and B
idA,B(H) = density of independent sets containing A and not B
• analogous properties to id(H) = id ,∅ ∅(H)
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Properties of idA,B(H)
1. For a vertex x outside A U B:
.
2. For a set W outside A U B:
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Out-sets
• for a given A, B, and any hyperedge S such that S∩B = ∅, the set S \ A is the out-set of S relative to A and B – example: A B
S• notation: idr
A,B(H) denotes that every out-set has cardinality at most r
• note that: idk,∅ ∅(H) = id(H)
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Claims
1. If A is not independent, then idrA,B(H) = 0.
2. If A is independent and there is an infinite family of disjoint out-sets, then idr
A,B(H) = 0.
3. If A is independent, then id0A,B(H)=2-(|A|+|B|).
4. Suppose that A is independent and there is no infinite family of disjoint out-sets. If O1, O2,…, Os is a maximal family of disjoint out-sets, then for all r > 0,
where W is the union of the Oi.
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Final steps…
• start with idk,∅ ∅(H)
• by (2), assume wlog there are finitely many out-sets• as A and B are empty, the out-sets are disjoint
hyperedges with union W• by (4):
• apply induction using (1)-(3); each term is rational, or a finite sum of terms where we can apply (1)-(3)
• process ends after k steps
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Unbounded rank
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…
H
Theorem (BBMP,14) 1. The independence density of H is
.
2. The value is S is irrational.
• proof of (2) uses Euler’s Pentagonal Number Theorem
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Any real number
• case of finite, but unbounded hyperedges• Hunb = {x: there is a countable hypergraph
H with id(H) = x}
Theorem (BBMP,14) Hunb = [0,1].
• contrasts with rank k case, where there exist gaps such as (1-1/2k,1)
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Independence polynomials
• H finite, independence polynomial of H wrt x > 0
i(H,x) = – for eg: id(H) = i(H,1)/2n
• example– i(Pn,x) = i(Pn-1,x) + xi(Pn-2,x),
i(P1,x)=1+x, (P2,x)=1+2x
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Independence densities at x
• (,x): defined in natural way for fixed x ≥ 0– may depend on chain if x ≠ 1– (,1) = id(H)
• examples: – (,x) = 0 for all x
• generalizes to chains with βn = o(n)
– (H,x) = 0 if x < 1:• bounded above by ((1+x)/2)n =o(1)
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Examples, continued
• +
• with chain (Pn: n ≥ 1), we derive that:
(,x) =
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Examples, continued
• for each r > 1, can choose chain C such that
) =
• r is a jumping point
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Future directions
• classify gaps among densities for given hypergraphs
• rationality of closure of set of id’s for rank k hypergraphs
• which hypergraphs have jumping points, and what are their values?
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General densities
• d: hypergraph function satisfying:– multiplicative on disjoint unions– monotone increasing on subgraphs
• d(H) well-defined for infinite hypergraphs
• properties of d(H)? for eg, when rational?
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